Many water treatment basins have unlined or incompletely lined bottoms and are subject to serious bottom erosion when downflow mixers and even some aerators are installed in the basin. ln order to reduce the erosion problem, it has been a common practice to provide a diffusion cone on the bottom of a downflow mixer such as disclosed in U.S. Pat. No. 4,422,771, to direct the liquid downwardly and outwardly in a generally conical stream. Such conical flow diffusers, however, impede mixing of the basin particularly in the area below the conical flow diffuser. It has also been proposed to provide an anti-erosion pad at the bottom of the basin below the mixer. To be effective in reducing erosion, the anti-erosion pad must cover a large area of the basin bottom below the mixer. Some anti-erosion pads have heretofore been formed of concrete that is poured in place on the bottom of the basin. However, such poured in place anti-erosion pads require that the basin be drained during installation of the pad and this is not always convenient or even possible in some basins. Moreover, mixers are frequently used in water treatment basins at locations too far from the boundary or shore of the basin to enable use of a shore based crane or the like to position a relatively large and heavy anti-erosion plate at the desired location on the bottom of the basin.